Specimen racks for receiving cuvettes are widely used in laboratory work. Some such specimen racks are also embodied such that with the cuvettes inserted, they can be passed through or inserted into a number of fixtures and measuring instruments without requiring that the cuvettes be individually removed.
The embodiment of such specimen racks in particular with inserted cuvettes in which the cuvettes remain even during the actual assay naturally depends on the physical nature of the particular assay involved, or in other words on the measuring instrument used on the one hand, and on the character of the radiation to be detected on the other. In specimen racks intended to receive cuvettes on which transmission measurements are intended to be performed, for instance, walls opposite the cuvettes are cut away in order to allow such measurements to be made.
The economy attained with batchwise assays of this kind, on the other hand, involves fundamental problems, however, because of the presence of the "ambient" field of the cuvettes, which is embodied by the material comprising the specimen rack.
On the one hand, the measurement sensitivity is sometimes impaired by the presence of such specimen racks. That is, because the wall thickness of such racks cannot be reduced below a certain minimum, the solid angle engaged by the particular measuring instrument necessarily drops, so that only a fraction of the radiation to be detected can actually be detected; this is particularly disadvantageous in detection processes involving small and/or only brief radiation amplitudes.
On the other hand, the proximity to one another of a plurality of cuvettes allows crosstalk to arise, which falsifies the results of measurement.
If an attempt is made to reduce these effects, for instance by using screens or collimators, that in turn can have negative effects on the measurement sensitivity.
These fundamental problems necessarily increase as the dimensions of the specimen rack and cuvettes decrease; in recent years, for various reasons (for instance to economize on expensive reagents), so called microtitration plates have gained a solid foothold in laboratory technology for performing the applicable radiation measurements; a certain standardization has been attained in terms of the dimensions of these plates, and a relatively large variety of laboratory equipment that fits them, such as automatic washers, is already available on the market. Microtitration plates are usually made in one piece as an injection molded part of plastic, having 8.times.12=96 specimen wells, or chambers. However, modular assembly of microtitration plates is also possible, by equipping a specimen rack with individual cuvettes or cuvettes assembled into strips, once again resulting in the same grid of 8.times.12 specimen positions.
For the reasons explained above, the problems of measurement sensitivity and crosstalk are exacerbated when such microcuvettes or microtitration plates are used, because of the immediate spatial closeness of the various cuvettes or specimen spaces. Therefore, as described in European Patent Document No. 00 56 417, the walls of the holder chambers can be lined with opaque material in order to eliminate the undesirable crosstalk effects, at least in these regions.
In that case, however, given the vertical measurement provided there, the bottom of the microtitration plate must be equipped with an optical window of transparent material, which is expensive for production reasons. The basic problem of crosstalk through such a measurement window is still not eliminated.
This is also true for the filling opening of the microcuvettes or microtitration plate: A great number of important detection methods, particular luminescence immunoassays, require the addition of a suitable reagent, for example by pipette, immediately prior to measurement, in order to induce the reaction to be detected. In practice, this means that an opaque upper covering of the filling openings cannot be provided sufficiently quickly, so that crosstalk from scattered radiation can still arise in this region.
The vulnerability of the various detection methods to the above-described impairments in measurement sensitivity, on the one hand, and to the disturbance caused by crosstalk on the other, depends once again on the nature and on the physical characteristics of the particular detection method, particularly on the intensity and duration of the radiation to be detected, or in other words on its amplitude course over time. For instance, in detection methods in which, because of this amplitude course, the total intensity of radiation to be detected is only slight, care must preferably be taken to attain great measurement sensitivity; but in detection methods offering a correspondingly wide variety of types of radiation, the problem of crosstalk gains greater significance, because even if the measurement of the various microtitration cuvettes is staggered in time, the disturbing radiation of adjacent cuvettes will not yet have faded sufficiently when the next ones are tested.
It has also been proposed in this connection to terminate the chemical reaction actively in one cuvette, before the next cuvette is measured. Such a procedure is disclosed in European Patent No. 0 148 736.
The embodiment of microtitration plates more or less in accordance with European Patent No. 00 56 417 requires a covering hood (limiter 13 in FIGS. 2 and 3 of that patent), which forms partitions between the recessed containers, in order to avoid such crosstalk phenomena.
It is also known to join a plurality of microcuvettes in strips with one another (making what are known as "strip-racks"), in which case, given a mechanically sturdy embodiment of the connecting regions, a microtitration plate that is reduced to a frame can be mounted in place, as described in European Patent No. 0 135 502.
Although this simplifies handling, the above-discussed problems of crosstalk between adjacent cuvettes may be aggravated still further, because the connecting regions may under some circumstances act as optical conductors. For luminescence assays, this design is accordingly unsuitable unless further provisions are made.